Transmission problem : 2nd Gear

[Cisco82]

Hi folks ,

today i had a bad surprise... i was driving and suddenly the 2nd gear disengaged without touching the gear level, i thought it was my fault but when i tried to engage the 2nd again i wasnt able to do it , the transmission was grinding and no way to engaged 2nd again.

So i tried to do the double shifting and , with a good acceleration in neutral position i was able to engage the second but , with the second engaged during deceleration it disengaged again.

I thought it was a synchronization problem but i think there is warn somewhere ... any idea or suggestion?


Thanks

[rickf]

Sounds like a bent shift fork, not fully engaging second gear. Don't see that very often but most times it is from people trying to shift from first to second far too fast and really pulling on the shifter. With that really long shifter that is a lot of leverage on the shift fork. You can take the transmission floor cover off and then take the top off of the transmission and look at the condition of the shift forks without taking the transmission out.

[Cisco82]

Thanow Rick, I ve removed the gear shift cover and i be checked the forks , i think the fork of second gear is the one toward truck rear but to be honest im not able to understand if it is bent or not cause i have no new one so...

I attach few pics, could give me your advise or suggest me a method to verify if the fork is the problem?

[rickf]

Francesco, You are correct that it is the rear fork. I cannot say from the pictures if it is bent or not. The pictures make it look a bit twisted but the shift collar it rides in is fine so I don't think that is an issue. Make sure the fork has not moved on the shaft. The gears do not look all that bad, I see a couple missing teeth on the fourth gear syncro but that will not affect second and probably not fourth. All I can suggest is to put the cover back on with just the four corner bolts and pull it into second gear as you normally would, no extra force. Then very carefully lift off the cover and see if that first straight cut gear is pulled all the way back onto the syncro and second gear. If it slides back a fair amount then that is your problem and you need to look for the reason the shifter is not pulling it all the way back. There are not too many places it can be. I do notice that the transmission had a lot of rust in it at one time and it was run that way and the rust pretty much wore off all the beveled gears. This is not a problem most of the time for the gears but be sure to check all of the bearings for roughness and play since they will be damaged by the rust. And I would suggest that while the transmission is out and opened up that you flush it out real well and put new oil in.

[Cisco82]

Thanks Rick , i ll check if the lever shift the gear in the correct position , but i have a doubt : you say the straight cut gear should be pulled completely back but if i correctly understood the gear lever movement the synchro assembly (the one toward the truck rear) should be pulled completely forward (toward truck front) if i engage the second gear, am i wrong?

However if i find the gear is not completely in position i suppose there are few options:
1 - Fork bent / loose / too wear -> fork or fork shaft to be replaced
2 - Wear into "fork housing" -> 1st-2nd gear synchro assembly should be replaced
3 - Main shaft wear ?? I dont know if it is possible...

I noted the rust and im aware it could damage the bearing , do you think it could be the problem? The play into rear / front bearings could give problems only for the second gear?

Thanks

F

[rickf]

You are correct on the direction, it needs to go in the direction of second gear, towards and over the syncro. I am in the middle of a bunch of paperwork right now so I will get back to you later with more info.

[rickf]

Ok, I am back. If it is not going fully into gear then I would say it has something to do with the shift fork. Could be the shifter rail where the shifter hits it but doubtful. 1-2 syncro is a no. There is no syncronizer for first gear and all the syncronizer does is stop the spinning gear set so they can engage which they are doing. So the syncro is doing it's job. I did notice that the 2nd gear syncro looks like it is worn judging by how close it is sitting to the gear, I think it was 2nd. But if they are not grinding don't worry about them. Mainshaft is not going to be the issue, if it were you have problems with other gears. But like I said flush it out very well and while it is clean and before putting in new oil spin thinks around and listen for rough bearings. Bad mainshaft bearings could possibly cause that problem, maybe. But they will definitely cause bigger problems down the road.

[Cisco82]

Thanks Rick , i ll check and i ll keep you updated.

However i was talking about 1st and 2nd gear assembly where there is the fork rail , may it could be worn out but i didnt find into 34 manual the maximum rail width.


The first thing im going to do is check the thickness of the fork , i ve read into 34 manual the min thk shall be 8.6 mm, lets see if it is into allowable limit.

Im going to check it during weekend , i hope it is only a fork issue , i really would like to avoid removing engine and tranny!!

Thanks again

F

[rickf]

Keep us posted, I would think that to be worn enough to not go all the way in gear it would be obvious. That has always been the case with ones I have worked on but measuring is a good start. If it does not push it all the way into gear that is something you will need to find the reason for. Unfortunately this is one of those cases where I really can't help much without actually being there to see and touch it.

[Cisco82]

Ok, I've checked the transmission , my observations as following

1 - The fork thk is close to the limit but i think few decimals cant make a big difference considering the following point
2- I ve engaged the 2nd gear then i ve removed slowly the cover and i ve checked the gear , it is not completely pushed toward truck front it can be shifted about 2 mm more till the gear touches the following one (see pics)
3 - I noticed there is a gap between syncro and the following gear(yellow one) with the same shape but different material (see pics , i ve also made a clip where i move the gear to give an idea of the play see following transfer link)

https://we.tl/t-TfdIigJkNO

Any suggestion?

The first thing i could try would be change the forks with NOS ones but im not able to find a vendor here in Europe with NOS ones ... I ve checked USA vendors but most of them doesnt ship to Europe or the cost is absolutely not affordable ... do you know anyone with a new set or gear forks?

20230601_180642.jpg

20230601_180606.jpg

20230601_175755.jpg

20230601_175737.jpg

20230601_175656.jpg

[rickf]

Ok, The gap between the brass gear and the steel gear is what makes the syncronizer work. The steel gear is a male cone and the brass gear is a female cone, When you shift those two are pushed into each other and the friction between them equalizes the speed between the gears so they will slide right together. That is how a syncro works. Note the one further back has a much smaller gap? it is worn. And I agree that the 2mm should not be making it pop out but I don't see anything else there that would cause it. If you push that shift fork and rail back and forth in the top cover does it snap into place for first and second gear? There are detent balls that have springs that push the balls into detents in the shafts to help hold the shifter in gear. If a spring broke it may not be holding the shaft in place. If the fork goes forward and pops into place and does the same in the other direction then the detents are good. Some transmissions do not have the detents, I am not in front of my manuals so I don't know about this one, I will look later when I get to that computer. I am pretty sure it has them because I have had to shift the cover into neutral before putting it back on a transmission and I know they can be stiff to get out of gear without that long shifter on there. This is all with the top cover off the transmission.

[Cisco82]

Yes Rick , i can engage the 1st and the second gear acting on the gear lever and it keep the position , when i removed the cover with the second gear selected the lever was fixed in that position , to change the position to neutral i had to reinstall the cover with the screws , i think the top cover has good dented balls.

I want to clarify a think : the last time i selected the second gear with engine off, then i switched on the engine and i tried to release the clutch pedal : the gear lever was fixed into 2nd gear (so dented balls are good i think), the gear didn't pop out but the gear was grinding hard , it looks like the gear wasn't contently into right position...

I was thinking if really 2 mm can make the difference , maybe i could try to cover the fork with a tape to increase the thk up to original dimension and check if it works ... however i think the video could give you more info... im really confused

[rickf]

I just watched the video and I did not really see anything out of order. The brass gear you were playing with is the syncronizer blocker ring and it is acting exactly like it should. It is a bit worn but I can see just by the way it binds tight against the steel gear it is working. At the very end of the video you shift out of fist towards second but yo do not go all the way into second, so I really can't tell anything there. The small amount of play in the second gear is just the gear moving on the main shaft between the third gear and the syncro snap ring and is normal. These are basically truck transmissions so they are not the tightest things in the world.

[Cisco82]

Problem solved!! After several tests the result was simple: the transmission is good but the screw that keep in position the handbrake wheel wasn't well tighten ( I know there is a lockwasher but it wasn't well secured) therefore it allowed the movement of the shaft a bit forward when I engaged the second with a grinding result.

Maybe it could be useful for other m151 fellows.

Many thanks to Rick, your support has been excellent as usual.

Regards

[rickf]

Francesco, Thanks for the follow up. Now that you mention that problem I do remember someone else having a similar problem many years ago. the gears in the transmission were getting chewed up real bad but I don't think it was jumping out of gear. But if that main shaft is moving it can create all kinds of issues. Good catch.